This invention relates to antenna circuit multiplexers for connecting a single antenna to several radio-frequency transmitting or receiving devices, and more particularly to radio frequency multiplexers constructed from a plurality of adjacent transmission line filters.
In radio communications systems, it is often desirable to operate transmitter and receiver devices simultaneously, in the same general location, and at frequencies relatively close to one another. In addition, it is often desirable to connect the transmitter and receiver devices to the same antenna.
For example, certain types of mobile radio systems include "repeaters," which receive a signal on a first frequency and simultaneously retransmit the signal on a second frequency for reception over a wide geographical area. Typically, the first and second frequencies may differ by 500 KHz to 30 MHz, depending on the application. Similarly, in cellular telephone systems and many other radio-based telephone systems, full duplex operation is required, so that both the subscriber equipment and the base station equipment must transmit and receive simultaneously on frequencies about 30 MHz apart.
In such applications, it is essential to prohibit significant amounts of the transmitted RF signal reaching the receiver equipment. At best, substantial signal levels at the transmitter frequency will degrade receiver performance by overloading the front end or intermediate frequency stages of the receiver, and at worst, such signals could damage the receiver equipment. At the same time, it is essential to permit signals captured by the antenna at the desired receive frequencies to reach the receiver equipment.
Accordingly, a variety of multiplexer devices have been developed in the past to allow several operating transmitters and receivers to be connected to the same antenna while preventing undesired signals from one transmitter from reaching the receivers or other transmitters. Such multiplexers are available in several configurations, but typically they comprise a plurality of relatively narrow, highly selective bandpass filters. Each filter is respectively associated with an individual one of the transmitters or receivers, or with a frequency band in which several such devices may operate. Further, each filter is typically constructed to define a pass band which substantially excludes the passbands of the other filters.
Thus, in a "duplexer" for connecting a transmitter and a receiver to one antenna, a transmit filter is tuned to a first frequency range in which the transmitter operates, and a receive filter is tuned to a second frequency range in which the receiver operates. The receive filter is interposed between the receiver and the antenna, and the transmit filter is interposed between the transmitter and the antenna. Since the transmitter operates in a frequency range outside of the passband of the receive filter, the receive filter effectively rejects any RF energy produced by the transmitter at the intended transmitter frequency.
Because it is impossible to construct a "perfect" transmitter, transmitters often generate spurious signals at various frequencies, including the frequency on which the receiver operates. However, because the receiver operating frequency range is outside the passband of the transmit filter, such spurious signals are substantially attenuated. Thus, the transmit filter and receive filter cooperate to allow a transmitter and receiver to share a single antenna, while substantially preventing signals from the transmitter from reaching the receiver. Where more than two devices must be connected to the same antenna (or other load), the basic duplexer design has been extended by increasing the number of filters.
As noted above, multiplexers, and the filters from which they are built, have been constructed in a variety of configurations. In one configuration referred to as an "interdigital" filter, a cavity of rectangular cross section is formed, and a plurality of spaced adjacent resonating elements are provided, each alternate resonator extending inward from an opposite wall of the cavity. In such filters, the resonant frequency of each resonator is determined by its length, the impedance of each resonator is determined by its cross sectional size, and the amount of coupling with other resonators is determined by proximity to the other resonators.
In the past, multiplexers have been conventionally designed with filters as separate units. Thus, in FIG. 1, which depicts a radio system 108 employing one conventional multiplexer (duplexer) design, a transmitter 110 is connected to a transmit filter 112, and a receiver 114 is connected to a separate receive filter 116. The transmit and receive filters 112, 116 are connected in parallel through transmission lines 118, 120 at a "T" junction 122, and then via a transmission line 124 to an antenna 126. Typically, each filter is independently designed (i.e., in isolation) for optimal performance, and thus, the transmission lines and other interconnecting components are preferably designed such that when the filters are interconnected to operate as a multiplexer, each filter has little or no effect on the performance of the other filters. However, it is believed that some multiplexers have been developed in which multiple filters are connected in a "T" or star junction configuration and in which the filters have been optimized for operation only in the grouped configuration.
Multiplexers of the design of FIG. 1 have several disadvantages. Providing separate filters, with separate filter enclosures and interconnections therebetween, increases the cost and space requirements of such multiplexers. In addition, these designs typically require that the transmission lines 118, 120 between the filters 112, 116 and the "T" junction 122 be cut to precisely defined lengths, often an even multiple of one-quarter wavelength at the filter center frequency. In some designs, the transmission lines may be intended to function as impedance transformers, or may be intended to isolate the individual filters. If the transmission lines are not manufactured to exactly the correct length, they may undesirably introduce reactance, or may undesirably transform the impedance of one filter as seen by the other filter. Thus, errors in the transmission line lengths can substantially degrade multiplexer performance. The transmission lines and associated connectors and other hardware are expensive, and the high expense and difficulty of manufacturing the transmission lines is exacerbated by the need to manufacture them to a precisely defined length. Further, the transmission lines take up valuable physical space in a radio system installation, and must be appropriately routed to avoid damage and to present a suitable appearance.
In order to solve some of these problems, multiplexers having several interdigital filters integrated into a single package have been developed. U.S. Pat. Nos. 4,596,969, and 4,660,004, issued to Ronald Jachowski, disclose multiplexers in which two or more interdigital filters are disposed in an integrated package in a side-by-side adjacent relationship. A prior-art multiplexer 140 of this type is shown in FIG. 2 so that the differences between it and multiplexers constructed according to the present invention may be more clearly illustrated. The prior art multiplexer 140 comprises a box-like conductive housing 146 of substantially rectangular cross-section for containing a transmit filter 142 and a receive filter 144. A conductive dividing wall 178 separates the transmit filter 142 from the receive filter.
The receive filter includes a plurality of resonators 174a-174e, a receiver coupling transformer 166, a tap lead 170, and a receiver connector 152. The transmit filter includes a plurality of resonators 176a-176e, a transmitter coupling transformer 168, a tap lead 172, and a transmitter connector 150. Each of the resonators 174, 176, has one free end and one fixed end. The fixed end is mechanically and electrically connected (or "grounded") to the bottom housing wall of the filter.
An inter-filter transformer 156 is provided which extends along the length of the housing and which has portions adjacent each of the filters for coupling radio frequency (RF) energy between the respective filters and the antenna. The transformer 156 has a first end which is electrically connected to the conductive housing wall and a second end which is electrically connected to the antenna connector 148. According to the Jachowski '969 and '004 patents, the electrical lengths of the portions of the transformer 156 for coupling RF energy to each adjacent filter should be selected to be approximately one-quarter wavelength at the center frequency of the respective filter. Further, the locations of these coupling portions should be selected so that each coupling portion begins at a position which is an even multiple of one-quarter wavelength at the respective filter center frequency from the grounded end of the transformer.
Thus, as shown in FIG. 2, the coupling section 158 of transformer 156, which is associated with the receive filter, begins at the grounded end of the transformer and extends for one-quarter wavelength (at the receive filter center frequency F.sub.r). The coupling section 162, which is associated with the transmit filter, begins at a position one-half wavelength from the grounded end of the transformer and extends for one-quarter wavelength (at the transmit filter center frequency F.sub.t). A spacing portion 160 of the transformer 156 separates the receive filter coupling section 158 from the transmit filter coupling section 152. Although the exact length of this spacing portion 160 depends on the difference in center frequencies of the filters, in practice, the center frequencies are relatively close to one another, so that the spacing portion 160 is about one quarter wavelength long.
The transformer 156 of FIG. 2 thus functions in a manner similar to the transmission lines 118, 120 and "T" connection 122 of the multiplexer of FIG. 1.
The transformer 156 of FIG. 2 couples both filters to the antenna transmission line, provides a transformation between the impedance of the antenna transmission line and the impedance of the filters, and provides a desired phasing or electrical separation between the filters so that the filters do not substantially affect one another.
As a result, each of the filters 142, 144 of FIG. 2 may be designed essentially independently, without concern that the electrical characteristics of one filter will modify the performance of the other filter when combined in an integrated assembly. See, for example, FIG. 3, which depicts in a stylized graph 210 the response of the filters over a range of frequencies. The abscissa 214 represents frequency; the ordinate 212 represents a response function proportional to the logarithm of signal strength. Curve 220 represents the response function of the receive filter over a range of frequencies about the receive filter center frequency F.sub.r 216. Curve 222 represents the response function of the transmit filter over a range of frequencies about the transmit filter center frequency F.sub.t 218. If a transformer is provided as taught in the Jachowski '969 and '004 patents, each filter's response curve remains substantially the same whether the filter is operated alone or in conjunction with the other filter.
Because of the high density of modern communications systems, such as cellular telephone systems, radio equipment for such systems must often be located at non-traditional sites in which space for such equipment may be extremely limited. Further, the manufacturers of radio equipment are under constant pressure from customers to reduce the size, weight, and cost of equipment. Accordingly, it is highly desirable to minimize both the amount of material required for housings and the physical space requirement of all equipment, including multiplexers.
Although multiplexers constructed according to the Jachowski '969 and '004 patents are believed to perform well, the requirement that the filter coupling portions of the transformer, and the associated filters themselves, be spaced approximately one quarter wavelength apart along the transformer makes it difficult to design highly space efficient multiplexers. Although it is desirable to have a certain amount of clearance between the filter resonators and the conductive cavity walls, the quarter wavelength separation required between filters according to the Jachowski '969 and '004 patents far exceeds the ideal clearance. Thus, in a duplexer of that design, as much as one-quarter to one-third of the physical volume may be considered excess.
An additional problem with conventional multiplexers is that certain prior art methods of adjusting the frequency of the resonators used therein have tended to produce a source of intermodulation. In many prior-art resonators, a frequency adjustment screw is provided,and its use produces intermodulation. Further, existing interdigital filter resonators use conventional metal forming techniques, and although their lengths can be adjusted during manufacturing, they generally do not include provision for adjustment of their length once manufacturing is complete. Typical resonators are plated with a highly-conductive coating prior to installation in a filter. Thus, metal forming techniques which require substantial deformation or removal of material are inappropriate because such techniques would damage the conductive coating.